Head Plate

ABSTRACT

The invention relates to a head plate, fixing the end of a tube bundle with a number of, in particular, porous tubes with a membrane in sealing manner. The head plate is made from a metal or a metal alloy with a melting point lower than the lowest failure temperature for a tube material and/or the membrane.

The invention relates to a head plate holding a tube bundle having aplurality of tubes with a membrane, in particular porous, at one end andsealing them tightly.

Tubes of said type, e.g., tubes, capillaries or the like made of aplastic, a plastic composite, optionally also with a laminar structure,made of a ceramic or a metal have a variety of applications.

For example, in the case of fuel cells, a fuel component is conveyed insuch tubes of a tube bundle, which is why such tubes designed as amicroreactor are regularly also provided with a membrane by which thefuel components reacting with one another are separated.

Another area of application of such tube bundles is for refining fuelsof biorenewable materials, e.g., bioethanol. Bioethanol flows through atube designed as a semipermeable membrane and the water present in thebioethanol is removed through the membrane.

In such tube bundles, there is fundamentally the problem of holding andsealing the tubes at one end. So-called potting or gripping of the endsof these tube bundles together and in particular also for a sealingconnection with a housing, a pipeline or the like is usuallyaccomplished by synthetic resin casting techniques, e.g., epoxy resins,although these are not without problems. For example, the differentthermal expansion coefficients at a working temperature of up to 150° C.in low-temperature fuel cells and/or up to 250° C. in medium-temperaturefuel cells lead to substantial thermal burdens, which can lead to morecracks, leakage and the like in the area of the head plate, whichusually results in failure of the entire apparatus.

Against this technical background, the object of the present inventionis to make available a head plate which can be designed to withstandhigh mechanical loads, to securely grip the individual tubes of the tubebundle and to hold them with a seal.

It has surprisingly been found that these technical problems are solvedby a head plate according to claim 1 in which the emphasis is on thehead plate being made of a metal or a metallic alloy of a lower meltingpoint than the failure temperature of the tube material and/or themembrane.

Low-melting metals, solders or mechanical alloys are usually extremelybrittle and coarse-grained after solidification of their melts. This isregularly associated with a great mechanical instability. Therefore,such materials are usually not suited for gripping the individual tubesof a tube bundle, in particular those of a small diameter, and sealingthem tightly.

Nevertheless, the present invention makes available such metals and/ormetallic alloys of a low melting point. This is also true in particularwith regard to the wide range of variation in diameters of the tubes ofthe tube bundle, said diameters often varying from less than 1 mm up tothe centimeter range.

With the choice of the metal or the metallic alloy, its melting point,which of course must be lower than the failure temperature of the tubeof material and/or that of the membrane but on the other hand is higherthan the operating temperature of a finished device, should be takeninto account. This can often be achieved if the melting point is between100° C., in particular 120° C., and 250° C.

The preferred metal for use is bismuth, chemical symbol Bi, or containsthe alloy bismuth.

Bismuth is a reddish white, shiny and moderately hard heavy metal. Likewater, bismuth contracts in volume on melting and expands by 3.32% onsolidification. Consequently, on cooling, bismuth and melts containingbismuth are excellent for penetrating into even the tiniest interspacesbetween the tubes of a tube bundle. In addition, bismuth has a very highchemical stability. For example, it is insoluble in nonoxidizing acids.

However, the melting point of pure bismuth is 271.3° C., which is toohigh for many applications. It is therefore preferable to use a bismuthalloy whose melting point can be lowered significantly when such asolution contains approximately 14% to 60% bismuth, 20% to 30% lead orup to 45% tin or antimony, cadmium, indium, zinc, tellurium, mercury orthallium.

With the applications mentioned in the introduction, however, it ispreferable for the alloy or the metal to be lead-free in particular.

A preferred alloy is a eutectic bismuth-tin alloy which has a meltingpoint of 138° C. and a density of 8.58 g/cm³.

The metals of a eutectic alloy are advantageously completely miscible ina molten state, and the melt solidifies like a pure substance at auniform temperature. On solidification, the components separate andcrystallize side-by-side in an extremely fine crystalline form, theeutectic structure. It is also advantageous that a eutectic has thelowest solidification point or melting point that is possible in thesubstance system in question, referred to as the eutectic temperature oreutectic point in the respective melting diagram.

A tube material will usually be a plastic, in particular a polymer,whereby the idea is in particular that a membrane may also be a polymermembrane. The failure temperature at which there is no longer anyfunctionality of the tube and/or membrane is often slightly less than200° C., so that such a polymer tube and/or such a polymer membrane willdefinitely not be damaged by the metal melt and/or the melt of thealloy.

Alternatively, a tube, in particular porous, may be made of a ceramicand/or a metal, in particular a sintered metal. Such tubes are mostsuitable for being provided with a layer of zeolite which forms amembrane. On the basis of their crystalline pore structure, suchmembrane layers are suitable for size-selective and shape-selectiveseparation of liquid and gaseous substance mixtures. In addition, thehydrophilic/hydrophobic hydrophobic character can be adjusted throughthe choice of the Si/Al ratio in the zeolite crystal. Thus a hydrophobiczeolite membrane can be made available for a selective separation oforganic solvents such as ethanol from water, with the help ofpervaporation at approximately 100° C.

In the case of tubes with a small diameter in particular, they may bebundled chaotically and in contact with one another, e.g., for refiningbioethanol. Nevertheless, due to the special properties of bismuth inparticular, the tubes are held securely and with a seal. However, forthe sake of safety, the axial length of a casting should be designed tobe longer than the diameter of an enveloping curve of a tube bundle.Then the individual tubes may be encased with a seal everywhere over theaxial length.

As an alternative, of course, the tubes of a tube bundle may also bealigned at a distance from a predefinable grid, as is customary withfuel cells, for example.

The invention will now be explained in greater detail on the basis ofthe drawings, in which exemplary embodiments are depicted schematically.The drawings show:

FIG. 1: a lateral view of a casting of the tube ends of tubes of a tubebundle, and

FIG. 2: an enlarged detail along line II, II in FIG. 1.

FIG. 1 shows a tube bundle 1 with a plurality of tubes 2 shown from theside, e.g., polymer membranes with an outside diameter of approximately0.5 mm. The tubes 2 are chaotically bundled and are touching oneanother. The outside contour of the tube bundle 1 is therefore merelyindicated by a dash-dot enveloping curve 3. It is nevertheless possibleto form a metallic head plate 4, which holds the individual tubes 2 atone end among one another and with respect to a housing, for example,and does so reliably and with a seal (see also FIG. 2).

To do so, a metal or a metallic alloy of the type defined in theintroduction is liquefied in a suitable pot shape and the tube bundle 1is immersed in the melt at one end. The tubes 2 may be sealed at one endin a known manner. However, this is not usually necessary with tubes 2having a small inside diameter, because the molten metal and/or themolten alloy penetrates axially into a tube 2 only to a slight extent.

To ensure that all the interspaces of the axial length of the head plate4 are also sealed, the axial extent of a casting 5 is usually designedto be greater than its diameter and/or than the enveloping curve 3.

The head plate 4 is manufactured by a cut, e.g., along line II, II, bymeans of which the tube ends are then exposed.

FIG. 2 shows a subsequently diagramed polished section of such a cut onan enlarged scale. Larger interspaces between the individual tubes 2 arehomogeneously sealed by a metal or a metallic alloy according to theinvention with extremely fine pores and homogeneously in this enlargeddiagram. If tubes 2 are extremely close to one another, optionally evenin contact, there remains a slight space (see arrow 6). However, thesespaces are reliably sealed by the axial length of the casting 5 and/orthe head plate 4 over the axial extent.

After appropriate machining of the lateral radial surface 7 of the headplate 4, the tube bundle 1 is then gripped with a seal and held at oneend for a further application.

1-13. (canceled)
 14. A head plate assembly comprising: a tube bundlehaving a plurality of tubes; and a head plate gripping ends of theplurality of tubes and sealing the same to one another; wherein the headplate comprises a metal or a metallic alloy material having a meltingpoint lower than a failure temperature of a material forming the tubebundle.
 15. The head plate assembly according to claim 14, wherein themelting point is between 100° C. and 250° C.
 16. The head plate assemblyaccording to claim 14, wherein the metallic alloy contains bismuth. 17.The head plate assembly according to claim 14, wherein the metallicalloy is a bismuth alloy.
 18. The head plate assembly according to claim14, wherein the metallic alloy is a eutectic bismuth-tin alloy.
 19. Thehead plate assembly according to claim 14, wherein the metal or themetallic alloy is free of lead.
 20. The head plate assembly according toclaim 14, wherein at least one of the plurality of tubes is made of apolymer.
 21. The head plate assembly according to claim 14, wherein atleast one of the plurality of tubes is made of a membrane.
 22. The headplate assembly according to claim 21, wherein the membrane is a polymermembrane.
 23. The head plate assembly according to claim 21, wherein themembrane is a zeolite membrane.
 24. The head plate assembly according toclaim 14, wherein at least one of the plurality of tubes is made of aceramic and/or a metal material.
 25. The head plate assembly accordingto claim 14, wherein the plurality of tubes are bundled chaotically toone another and in contact with one another.
 26. The head plate assemblyaccording to claim 14, wherein the plurality of tubes in the tube bundleare aligned with a distance between them on a predefinable grid.
 27. Thehead plate assembly according to claim 14, further comprising a castingfrom which the head plate is formed, the casting having an axial lengthlonger than a diameter of an enveloping curve defined by the tubebundle.
 28. The head plate assembly according to claim 14, wherein theplurality of tubes are porous.
 29. The head plate assembly according toclaim 14, wherein the metal comprises bismuth (Bi).